popper-logic-scientific-discovery

have, after all, allowed psychologism to slip back quietly into my
theory. But this is not so. Admittedly, it is possible to interpret the
concept of an observable event in a psychologistic sense. But I am using it
in such a sense that it might just as well be replaced by ‘an event
involving position and movement of macroscopic physical bodies’. Or
we might lay it down, more precisely, that every basic statement must
either be itself a statement about relative positions of physical bodies,
or that it must be equivalent to some basic statement of this ‘mechanistic’
or ‘materialistic’ kind. (That this stipulation is practicable is
connected with the fact that a theory which is inter-subjectively testable
will also be inter-sensually 1 testable. This is to say that tests
involving the perception of one of our senses can, in principle, be
replaced by tests involving other senses.) Thus the charge that, in
appealing to observability, I have stealthily readmitted psychologism
would have no more force than the charge that I have admitted mechanism
or materialism. This shows that my theory is really quite neutral
and that neither of these labels should be pinned to it. I say all this only
so as to save the term ‘observable’, as I use it, from the stigma of
psychologism. (Observations and perceptions may be psychological,
but observability is not.) I have no intention of defining the term
‘observable’ or ‘observable event’, though I am quite ready to elucidate
it by means of either psychologistic or mechanistic examples. I
think that it should be introduced as an undefined term which
becomes sufficiently precise in use: as a primitive concept whose use
the epistemologist has to learn, much as he has to learn the use of the
term ‘symbol’, or as the physicist has to learn the use of the term
‘mass-point’.
Basic statements are therefore—in the material mode of speech—
statements asserting that an observable event is occurring in a certain
individual region of space and time. The various terms used in this
definition, with the exception of the primitive term ‘observable’, have
been explained more precisely in section 23; ‘observable’ is
undefined, but can also be explained fairly precisely, as we have seen
here.
1 Carnap, Erkenntnis 2, 1932, p. 445.
the problem of the empirical basis 85